Abstract: An uninterruptible power supply includes removable individual power modules, which for example may be 10 kilowatts each and are removable from the overall system without adversely affecting the remaining modules. Each module by means of pulse width modulated switching converts the line AC to DC where it is stored by use of capacitors or a battery and then inverted from DC back to AC. This is done on a three-phase basis without need of any low frequency transformer connection. Common input and output control units provide for the basic pulse width modulated switching signals and other necessary signals.The efficiency and speed of the system is enhanced by the use of cascode switches, saturable cores in transformers and inductors, and a proportional base drive to the switching transistors.